1. Field of the Invention
The invention relates generally to monitoring fluid flow, and more particularly, to detecting upstream fluid conditions in an intravenous fluid administration system.
2. Description of Related Art
Fluid delivery systems for infusing parenteral fluids are commonly used in hospitals and typically include an inverted bottle or bag or other means of supply of parenteral fluid, an intravenous (IV) administration set that is secured to the supply of parenteral fluid and includes a flexible IV tube, a cannula that is mounted to the distal end of the flexible IV tube and that is adapted to be inserted into the patient""s blood vessel to thereby infuse the parenteral fluid, and an infusion pump. Such infusion pumps provide a positive means for controlling the amount of fluid administered, and are an alternative to gravitational flow systems. In many cases, the pump is a peristaltic type in which a plurality of fingers, rollers, or other devices sequentially constrict along a moving zone of occlusion the flexible IV tube through which the parenteral fluid is supplied.
A common problem facing infusion systems is the evaluation of the condition of the fluid supply system upstream of the pump. For example, where an occlusion of the tube exists upstream of the pump, the pump will not succeed in infusing the parenteral fluid to the patient even though the pump may continue to operate. Similarly, where the parenteral fluid supply becomes depleted, once again the pump may continue to operate but no parenteral fluid will be delivered to the patient.
A prior method for detecting depletion of the fluid supply or an upstream occlusion was visual observation. A drip chamber may be inserted in the fluid line at a position downstream from the fluid supply for monitoring the rate and quantity of fluid administered. However, visually verifying the existence of drops requires the time of an attendant which can be an undesirable burden on the hospital staff.
In infusion systems utilizing peristaltic pumps, detection of upstream occlusions has been accomplished through the use of an opto-electric drop detector combined with a drip chamber. The drop detector automatically detects upstream occlusions, such as occlusions caused by a clamp or kink in the upstream tubing, by detecting an absence of drops. However, movement of the IV administration set, if severe enough, can cause extra drops to fall from the drop former or can interrupt the drops, causing false counts and false alarms. Ambient light can also interfere with an optical drop sensor and render it less accurate.
Another method of detecting upstream occlusions involves the addition of a pressure sensor to the fluid line upstream of the pump. However, the use of these devices can add considerable cost to both the instrument and the disposable portion of the IV administration set.
Another method for detecting upstream occlusions is to incorporate a pressure sensor into the pumping mechanism of the infusion pump itself. In one such device, a pressure transducer is placed in the middle of the pumping area, allowing direct measurement of the pressure in the pump tubing segment, which is indicative of the inlet pressure. However, this can adversely affect flow uniformity and may require substantial modifications to the pumping mechanism.
Pump systems have been disclosed that include a downstream pressure sensor used for detecting improper fluid communication with the patient. Such systems include U.S. Pat. No. 4,743,228 to Butterfield; U.S. Pat. No. 4,460,355 to Layman; U.S. Pat. No. 4,534,756 to Nelson; and U.S. Pat. No. 5,356,378 to Doan
In operation, peristaltic pump mechanisms sequentially occlude the pumping segment of the tube, also known as the pumping control segment, to alternately expose the pumping segment to fluid communication with the upstream and downstream portions of the fluid line. The pumping segment is at upstream pressure when exposed to the upstream portion of the fluid line. When the pumping segment is subsequently exposed to the downstream portion, the fluid within the pumping segment, which was at upstream pressure, causes a change in pressure, i.e., a pressure difference, as the pumping segment pressure equalizes with the downstream portion.
Some pump systems using downstream pressure sensors have utilized analysis of such pressure differences to detect upstream occlusions. If a large negative pressure difference occurs, an upstream occlusion is presumed. However, pumping into high downstream pressures can create pressure waveform conditions, including drops in pressure, that mimic the appearance of true upstream occlusions. Additionally, pressure sensors may exhibit substantial offset errors that can also mimic upstream occlusion conditions. Pressure sensors used with IV systems may themselves cause some variance and their readings can vary substantially. Such variances, which may be produced by temperature differences or other factors, can cause false alarms. In some cases, variances can be reduced through compensation circuits or closer tolerances on various mechanical and circuit elements. However, this approach can add substantial expense.
Utilizing an existing downstream pressure sensor to determine upstream fluid conditions can result in less expense both in the pump itself as well as in the entire IV administration set. However, false alarms can erode the usefulness of an occlusion detection system. Accordingly, it is desirable to use a single downstream pressure sensor having less stringent accuracy requirements while avoiding false alarms.
Hence, those skilled in the art have recognized the need for a fluid line monitoring system that can automatically detect upstream fluid line occlusions while mininizing false alarms. Additionally, those skilled in the art have recognized a need to reduce the cost of a system capable of determining such upstream fluid line conditions. The present invention fulfills these needs and others.
Briefly and in general terms, the present invention provides a system and a method for detecting a condition in an upstream portion of a fluid line coupled between a fluid supply at its upstream (i.e., intake) end and a fluid receiver at its downstream (i.e., outlet) end, comprising a flow control device coupled to a fluid line segment of the fluid line between the upstream and downstream ends for alternately opening the fluid line segment to fluid communication with the upstream end and the downstream end of the fluid line. A pressure sensor senses pressure in the fluid line and provides a pressure signal in response to said pressure sensing. A processor monitors the pressure signal to determine the pressure difference occurring when the fluid line segment is exposed to the pressure from one end of the fluid line after having been exposed to the pressure at the other end of the fluid line. The processor averages the pressure signal, and based on the pressure difference and the pressure average, the processor provides a condition signal indicating a fluid line condition in said upstream portion of the fluid line.
The fluid pressure difference is the temporary change in downstream pressure that occurs at the time when the pumping control segment having fluid at the head (i.e., intake or upstream) pressure is opened to fluid communication with the downstream end of the fluid line. When an upstream occlusion (xe2x80x9cUSOxe2x80x9d) occurs, the upstream pressure typically becomes quite low and may form a partial vacuum. When the pumping control segment at low upstream pressure is placed in fluid communication with the downstream portion at higher pressure, fluid rushes from the downstream portion into the pumping control segment, thus causing a temporary pressure drop in the downstream pressure. The magnitude of this pressure drop is the downstream pressure difference.
The current invention discriminates false upstream occlusion indicators by comparing the average pressure to a threshold pressure, with the threshold pressure being a variable determined from the pressure difference.
The invention monitors the downstream pressure over a period of time, such as a single revolution of the peristaltic pump mechanism, to determine an average downstream pressure. The system uses the pressure difference to determine a threshold pressure value. The system compares the average pressure to the threshold pressure to detect an upstream occlusion. In a further aspect, the system activates an occlusion alarm if one or more sequential revolutions of the pump mechanism result in average pressures that exceed threshold pressures.
In a further aspect of the invention, the pressure difference is used to determine two threshold pressures: a first or primary threshold pressure and a second or cautionary threshold pressure. If the average downstream pressure is greater than either of the two threshold pressures, no occlusion is indicated. Where the average pressure is less than the primary threshold pressure, an upstream occlusion is indicated. The second or cautionary threshold pressure, which is typically larger than the primary, is used to indicate a potential upstream occlusion.
If the average pressure is less than the cautionary threshold pressure but greater than the primary threshold pressure, the system presumes that the presence or absence of an upstream occlusion is indeterminate. In such a situation, the system initiates a confirmation test to determine the presence or absence of an upstream occlusion. Such a test typically involves a temporary reversal of the pumping mechanism for a specific portion of the mechanism""s rotational cycle. For each temporary reversal, the new pressure difference is monitored and compared against a post-reversal cutoff. Depending on the comparison of the pressure difference obtained after the reversal to the cutoff, the system may adjust the cautionary pressure threshold thereby adapting the system to lessen the chances of false alarms.
Other aspects and advantages of the invention will become apparent from the following detailed description and accompanying drawings, illustrating by way of example the features of the invention.